This invention relates to magnetic resonance imaging apparatus.
Magnetic resonance imaging (MRI) is used in medicine to produce images of the internal organs of a patient being examined. In MRI a static magnetic field is applied to the body of the patient to define an equilibrium of magnetic alignment in the region of the body being examined. A radio frequency field is then applied to the region being examined in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in the region. This resonance produces signals in r.f. coils placed adjacent the patient's body.
Within this general principle of operation a wide of variety of apparatus designs have evolved to meet a large number of different requirements. This means that whilst a particular design of apparatus is very suitable for one type of application it is unsuitable for another. Consequently if a user, such as a hospital, wishes to be in a position to provide a wide variety of MRI based facilities/services/treatments then it would have to become involved in the purchase of a number of different tailor-made MRI systems.
In designing an MRI system there are a variety of mutually conflicting constraints. One such constraint is the conflict between providing a high magnetic field strength on the one hand and patient accessibility on the other. Another constraint concerns the weight of the field magnetic assembly where, for example, the equipment is to be installed in an upper floor of a building, MRI equipment being very heavy because of the employment of massive high powered magnets.
Therefore, in designing MRI equipment design compromises have to be struck or if one aspect of the design is given priority there will be consequential drawbacks in other areas.
The present invention is concerned with resolving one of these design conflicts where the weight of the MRI equipment has to be kept below a certain level thus limiting the main magnetic field to a relatively low value and yet it is desired to use the equipment to carry out certain activities/functions (e.g. spectroscopy) where it is necessary to have a magnetic field which is higher than that which can be generated by the earlier mentioned weight-constrained main magnetic system.
To produce a high field sufficient for spectroscopy would be impracticable with a large bore magnet but would be practicable with a small bore magnet. A problem with such a high field magnet would be the question of shielding, particularly if used in the vicinity of a magnet for magnetic resonance imaging.
Active shielded magnets have been proposed, in which the high field magnet is provided with an outer set of windings, surrounding the main windings for generating the high field. The outer windings are designed to produce zero field on the outside of the windings, because they oppose the field from the high field magnet in that region.
In conventional actively shielded magnets, the two sets of windings are contained inside a common cryostat.